* Avoid a context switch in the history api
The history api was creating a job to fetch the
states and another job to convert the states to
json. This can be done in a single job which
decreases the overhead of the operation.
* Ensure there is only one sqlalchemy session created per history
query.
Most queries created three sqlalchemy sessions which was
especially slow with sqlite since it opens and closes the
database.
In testing the UI is noticeably faster at generating history
graphs for entites.
* Add additional coverage
* pass hass first to _states_to_json and _get_significant_states
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